Among operation detecting devices of a clamp, there is a device including: a clamp rod axially movably inserted into a housing; a detection hole formed in a lower end wall of the housing, substantially concentrically with an axis of the clamp rod; and a detection member axially movably fitted into the detection hole (for example, see Patent Document 1). On an outer peripheral surface of the detection member, there are provided: a closing surface which closes an opening portion of an inlet hole for supplying pressurized fluid that is opened in a peripheral surface of the detection hole; and a recessed portion through which the opening portion is communicatively connected to outside air. When pressurized fluid is supplied to the inlet hole, there is a difference, in pressure thereof, between a case where the opening portion of the inlet hole faces the recessed portion of the detection member and a case where the opening portion is closed by the closing surface, and therefore, through detection of the pressure of the pressurized fluid, the operation detecting device detects an operating state of the clamp rod.    Patent Document 1: JP2003-305626A